By Caroli Geldenhuys & *Richard Walls (Pr. Eng.), Stellenbosch University, Dept. of Civil Engineering, Fire Engineering Research Group.
Passive protection such as intumescent paints, vermiculite boards and spray-on products can be very expensive. Thus, rational structural fire design methods, as presented here, can lead to significant savings. This article presents a brief introduction to a design method, the Slab Panel Method (SPM), which allows engineers to design composite floors for fire.
Believe it or not, when you set fire to a composite steel and concrete floor it just doesn’t want to fall over. It heats up to hundreds of degrees Celsius, beams buckle, floors sag, concrete can crack – but floors don’t collapse because they become giant hanging catenaries. Using results from full-scale tests this behaviour can now be modelled and designed for, potentially leading to significant savings in the cost of passive fire protection on steelwork – with as much as 40-50% of floor beams not needing protection (see Figure 1). Passive protection such as intumescent paints, vermiculite boards and spray-on products can be very expensive. Thus, rational structural fire design methods, as presented here, can lead to significant savings. This article presents a brief introduction to a design method, the Slab Panel Method (SPM), which allows engineers to design composite floors for fire.
The Slab Panel Method is a structural fire design method used for composite steel and concrete floors in severe fires. Certain steelwork can reach above 850°C in the design, but the system still remains structurally sound. The method was developed by Prof Charles Clifton in New Zealand.
See the report R4-131:2006 for complete details regarding calculations. His work was originally based on the results of the famous full-scale fire tests done at Cardington by the BRE (Building Research Establishment), and the tensile membrane work by Prof Colin Bailey. Since then the SPM has been developed based on numerous other research projects as well. Figure 2 shows the eight storey composite steel and light-weight concrete building used at Cardington, where various parts of the building were progressively exposed to severe fires and the overall structural behaviour studied.
The SPM procedure incorporates the reserve strength from a floor system under deformation in a fully developed fire. It is an ultimate limit state design procedure in some ways similar to building design for response to earthquakes, in that certain degrees of structural damage are permitted provided that collapse is prevented, but damage may occur in very severe fires.
How the SPM Works
The SPM design model is based on using yielding-moment action and tensile membrane enhancement. The procedure is applied to large regions of a floor, known as slab panels, and incorporates the inelastic response of slabs (i.e. floors bend and sag permanently). At ambient temperatures the way loads are transmitted through a composite steel building involves; The slab -> secondary beams -> primary beams -> columns.
When severe fire conditions occur and the interior secondary beams are unprotected, they lose most of their strength and the load path above cannot be maintained. The beams form plastic hinges and the load-carrying mechanism changes to a two-way spanning system, as illustrated by Figure 3. Here the load carrying path becomes:
The slab panel -> primary supporting beams – > columns.
From this, it can be seen that the secondary beams no longer play a major role, and simply form part of the sagging slab panel system. Hence, they do not need to be passively protected. However, it is essential that primary beams are protected as these carry the sagging slab panels.
The SPM theory is based on membrane action which is caused by in-plane forces within the slab. This allows the composite floor slab to bridge over the unprotected beams. This basically means that the rebar in the concrete slab, and the remaining secondary beams’ capacity, allow floors to hang from where support can be found, even when steelwork has failed. Figure 4 shows displacements of the beams that occurred during the fire tests at Cardington. It can be seen that no structural collapse occurred, even though significant deformation has occurred. It is interesting to note that the structure shown in Figure 4 had no passive protection whatsoever, experienced temperatures of over 1000°C, should have failed according to any standard design codes and yet did not collapse.
Under ultimate load conditions at ambient temperature, yield-line behaviour develops first and then tensile membrane enhancement, which occurs as the plastic hinges form. But under severe fire conditions, tensile membrane enhancement occurs first – i.e. the floor capacity increases as it becomes a hanging catenary. In the event of a fully developed fire, the SPM performs as follows:
1. The slab and the unprotected secondary beams may undergo considerable permanent deformation.
2. The primary support beams and columns undergo much less permanent deformation compared to that within the panel.
3. The load-carrying capacity and the integrity of the floor system are preserved.
4. Both local and global collapse are prevented.
5. The development of the failure patterns in a slab panel is shown in Figure 5. The design equations are based on the final layout shown in Behaviour Mode (iv), as also seen in Figure 3.
After a severe fire secondary beams may potentially need to be repaired or replaced. However, very severe fires cause such significant damage, that everything in the building would probably have been destroyed. Recently a structure designed according to the SPM experienced a fire, and the structure survived with almost negligible damage. It is understood that the cost of the damage to the contents far exceeded the cost of the damage to the structure.
Software for the SPM
The Heavy Engineering Research Association of New Zealand has developed software to do the numerous calculations required to carry out SPM designs. This can now be purchased from Steel Construction NZ. Alternatively, similar free software that could also be used for this purpose is MACS+ from ArcelorMittal or TSLAB from the SCI in the UK. Stellenbosch University is currently using the SPM software for research purposes.
Note: before using any of the above software make sure that you read and understand the design theory and methodologies. Rebar and certain detailing requirements are essential for the use of these methods, and the SPM guidelines provide good information regarding this.
The SPM and fire design in South Africa
SANS 10400 – Part T states that rational fire design in South Africa may be used provided that it achieves the same level of safety as implied by the document. These rational designs must be in accordance with BS 7974, which further states that competent persons must demonstrate that due diligence has been applied during the design process and the approving authorities can assess that due diligence has been applied. This basically means that under the auspices of rational design methods such as the SPM could be applied safely and according to SA code requirements (but it is important to discuss this with your local fire chief and fire engineers).
A recent project at Stellenbosch University (Geldenhuys, 2014) sought to calibrate the SPM to suit local South African conditions. It was shown that the SPM can be used as is, with only minor adjustments where New Zealand’s fire loading code is included. For more information on structural fire engineering principles and design, you can contact the *corresponding author at Stellenbosch University’s fire engineering research group.
South Africa will soon have additional fire design principles and methods available in the updated SANS 10162-1 structural steel code. The recommendations presented in the Canadian CSA S16 code will be adopted, making fire design available to local engineers in the near future. More details to follow soon.
*Corresponding author: firstname.lastname@example.org, 021-808-9584